The ambassador, Dmitry Rogozin, posted the message in response to recent reports that Moscow's Cold War ally Romania had agreed to host interceptor missiles as part of a revised US missile defence system.
"The Americans and their allies again want to surround the cave of the Russian bear?" Rogozin tweeted, along with a link to a news report about the Romania decision.
"How many times must they be reminded how dangerous this is!? The bear will come out and kick the ass of these pathetic hunters," he added in his next tweet, writing in Russian.
-- Luke Allnutt